The Vtc proteins in vacuole fusion: coupling NSF activity to V0trans‐complex formation

The fusion of cellular membranes comprises several steps; membrane attachment requires priming of SNAREs and tethering factors by Sec18p/NSF ( N ‐ethylmaleimide sensitive factor) and LMA1. This leads to trans ‐SNARE pairing, i.e. formation of SNARE complexes between apposed membranes. The yeast vacuole system has revealed two subsequent molecular events: trans ‐complex formation of V‐ATPase proteolipid sectors (V 0 ) and release of LMA1 from the membrane. We have now identified a hetero‐oligomeric membrane integral complex of vacuolar transporter chaperone (Vtc) proteins integrating these events. The Vtc complex associates with the R‐SNARE Nyv1p and with V 0 . Subunits Vtc1p and Vtc4p control the initial steps of fusion. They are required for Sec18p/NSF activity in SNARE priming, membrane binding of LMA1 and V 0 trans ‐complex formation. In contrast, subunit Vtc3p is required for the latest step, LMA1 release, but dispensible for all preceding steps, including V 0 trans ‐complex formation. This suggests that Vtc3p might act close to or at fusion pore opening. We propose that Vtc proteins may couple ATP‐dependent NSF activity to a subset of V 0 sectors in order to activate them for V 0 trans ‐complex formation and/or control fusion pore opening.